Academic
Comma which: 82,741
[noun] which: 31,585
(2.6 to 1 ratio)
Fiction
Comma which: 42305
[noun] which: 10810
(3.9 to 1 ratio)
Magazine
Comma which: 95265
[noun] which: 7773
(12.3 to 1 ratio)
Newspaper
Comma which: 92,741
[noun] which: 5,392
(17.2 to 1 ratio)
So it looks like academic writing is the most common place to find people using which without a comma. This is very counter-intuitive and very interesting.
-Bill